|Early diagram trying|
twin mirrors at foot
In the section the airgap is behind the upper mirror, so perhaps rain is able to run down the back too.... Rain? What rain? we haven't seen any since March!
When building the boxes I had the idea that ventilation of the boxes would help, but as I have used them for over a year, it has become apparent that for the greatest part of the time, they are better being airtight. They never overheat, the temperature gets to a maximum of mid forties, and that is quite safe - as long as the black chillers work, they are drawing heat away and this never exceeds low twenties, because the heat is being dumped.
I am not sure what the sunbox air temp would be if I turned the pump off on a very hot day. If there is a temporary shut down due to a faulty pump, energy meter or thermostat, it could go up to 50º but I have vents top and bottom to release the heat.
I will try to source some compressible foam to put up there, and then check the resulting temperatures. A more permanent fix would be some thin gauge aluminium to put across the gap.
This section through the boxes shows the upper mirrors, and their 60º angle. Ignore the lower mirrors, I decided after the upper ones worked well to make the foot mirror simpler and single. The dotted blue lines in the diagram illustrate the solar midday angles for summer, equinox and winter.